1. Field of the Invention
This invention relates to a color picture tube, with support members for the mask frame and more particularly to a color picture tube with a shadow mask structure comprising a shadow mask and a mask frame suspended at the diagonal portions of an envelope panel.
2. Description Of the prior Art
In general, a shadow mask structure of a color picture tube is suspended by support members to be engaged with stud pins implanted at the diagonal corner portions of inner sidewalls of the panel.
For example. U.K. Patent No. 1,189,403 discloses a shadow mask structure suspended through four support members on the four corners of the substantially rectangular panel. There are several advantages to this type structure. First, since the substantially rectangular mask frame is suspended by its four corners, the influence of deformations of the mask frame is smaller in comparison with a structure in which the mask frame is suspended by the center portions of the panel side. This can reduce electron beam misregister on phosphor elements of an associated phosphor screen. Second, for the same reason, electron beam landing misregister caused by vibration can be reduced. Thirdly, so-called long-term color purity drift phenomena which occur 30 minutes or more after initial tube operation, can be corrected without the use of bimetal that has been commonly used. The principle of this correction will be described with reference to FIG. 17.
In FIG. 17, a support member 25 is secured to the sidewall of a mask frame 20. For convenience of manufacturing, a plate 2 is often interposed between support member 25 and mask frame 20 for welding to mask frame 20. When an aperture 22 formed by thermal expansion on the shadow mask 23 shifts toward the periphery (from the dashed line to the solid line) to a position 24 as indicated by the arrow, support member 25 having an angle .theta. with respect to a tube axis parallel line 26 functions so as to move aperture 22 toward the phosphor screen to a position 27. Thus, the path of the electron beam 28 does not change and electron beam landing misregister does not occur. For this purpose, the angle .theta. is usually selected at a substantially right angle to the path of an electron beam 28 reaching the screen corner. For example, in the case of a 90-degree deflection tube, the angle .theta. is approximately 45.degree..
An angle .theta. of a 110-degree deflection tube, may be selected appropriately at 35.degree.. However, as shown in FIG. 17, in order to install the shadow mask structure properly on a panel 29, it is necessary to leave a space S between the extended portion 25a of support member 25 and the sidewall 20a of frame 20. When the angle .theta. is smaller, an inclined section 30 of support member 25 has to be longer. As a result, the resistance of the structure to mechanical impact is reduced. An increase of the angle .theta. leads the excessive correction of the purity drift.
Recently a color picture tube with a shadow mask having a small thermal expansion coefficient, such as invar , i.e., a 36% Ni-Fe alloy having a thermal expansion coefficient of approximately 1.2.times.10.sup.-6 /.degree.C., and a mask frame of iron has been developed. The use of the above-described support member, however, results in the occurrence of electron beam landing misregister. The reason can be explained as follows. When a temperature rise within the tube occurs, expansion of the shadow mask 23 effectively is avoided. Thus, the aperture 22 does not shift as shown in FIG. 18. On the other hand, the mask frame 20 is made of iron having a thermal expansion coefficient of approximately 10 times that of 36% Ni-Fe alloy (i.e., approximately 1.2.times.10.sup.-5 .degree. C. at room temperature). Thus, the mask frame 20 exhibits thermal expansion. As a result, the support member 25 causes the shadow mask 23 to move toward the phosphor screen 31, as shown by the solid line in FIG. 18. The aperture 22 is moved to a position 24. Consequently, the path of the electron beam passing through the aperture changes from the position 28 to the position 32, and the electron beam becomes misregistered. If the shadow mask 23 and the mask frame 20 each is made of a material having a small thermal expansion coefficient, such as invar, and the thermal expansion of the panel 29 has no expansion, such problems can be avoided. However, this causes a significant increase in the manufacturing costs, and is not suitable for practical use.
As described above, when the conventional support members are used, electron beam landing misregister occurs. Consequently, long-term color purity drift, and mechanical weakness result.